1. Field of the Invention
This invention relates to an impact-resistant resin composition which is capable of being powder-molded even under low-shear conditions to produce molded products of high impact strength.
2. Description of the Prior Art
Unlike the extrusion or injection molding processes, the conventional powder-molding process involes fusing resin powder under conditions of low shear. Some processes include the known rotary molding process, the rock-and-roll process, the Heisler process, the Engel process and the sinter process.
Powder-molding is most suitable for molding large sized articles and hollow articles having an intricate shape. It is economical and versatile in its applicability to the production of products of a variety of shapes and in small quantities. For these reasons the method is coming into more general use.
Plastic materials suitable for use in powder molding are polyethylene resins, polyvinyl chloride resins, nylon resins, polycarbonate resins, polyacetal resins, polystyrene resins, high impact polystyrene resins and ABS resins.
All the above materials require careful operation in the manufacture of articles by powder molding. For instance, in rotationally molding ABS resin in the form of powder of general purpose grade into products of good appearance, care must be taken to regulate the resin's particle size, to use precisely designed molds, and to control the temperature strictly, so as to obtain products of acceptable appearance. Even when this careful molding is followed, the product is still liable to be brittle and to have insufficient impact strength, compared with that originally possessed by the ABS resin. Accordingly, there is a need to improve the physical properties of such molded articles intended for practical use.
Accordingly, the inventors have conducted research in the field of powder molding ABS resin seeking to discover a composition having good moldability and high impact resistance. Our prior discoveries concerning this are disclosed in United States patent application Ser. No. 378,255, filed July 11, 1973, now abandoned and replaced by continuation application Ser. No. 566 787, filed Apr. 10, 1975, the entire contents of which are incorporated herein by reference.
According to that invention we had discovered that a mixture of a vinyl aromatic monomer, vinyl cyanide monomer, and a butadiene rubber component together with a straight chain saturated polyester, when polymerized by the two-stage bulk and suspension polymerization processes, gives a rubber-modified ABS resin having improved impact resistance, and furthermore, the final ABS resin composition, when powder-molded, provides molded articles having generally acceptable physical properties and improved strength.